FEL2004

1. René Bakker rene.bakker@psi.ch FEL2004 Beam Stability Issues

2. Proceedings will become available through JACoW:http://www.jacow.org • Preliminary proceedings available on conference web-site:http://www.elettra.trieste.it/fel2004/proceedings.html(papers & slides of oral presentations) • The abstract booklet, author and affiliation index is available at:http://www.elettra.trieste.it/fel2004/abook.html

3. Types of free-electron lasers: • Single-pass devices • Storage-ring FELs • High average-powerdevices • I will just discuss some examples, for an extensive view, please visit: • the FEL2004 web-site: http://www.elettra.trieste.it/fel2004 • FEL virtual library: http://sbfel3.ucsb.edu/www/vl_fel.html

4. Types of free-electron lasers: • Single-pass devices • Storage-ring FELs • High average-powerdevices

5. Types of free-electron lasers: • Single-pass devices • Storage-ring FELs • High average-powerdevices

6. acceleration deceleration Types of free-electron lasers: • Single-pass devices • Storage-ring FELs • High average-powerdevices FEL + ERL: Energy Recovery Linac Concept

7. Types of free-electron lasers: • Single-pass devices: • Storage-ring FELs: • High average-power:devices • high power / short wavelengths • VUV  X-ray • optical properties / spectrum • visible  VUV • IR

8. TTF / XFEL 4GLS MIT / Bates LEUTL PAL – FEL LUX SCSS BESSY FEL LCLS LEG FERMI VISA / DUV SPARC / SPARX Single Pass FEL Activity SC technology / NC technology

9. BINP / VEPP3 4GLS UVSOR JAERI FEL New SUBARU DFEL SACO IR-FEL EUFELE Storage Ring & High Average Power FELs No longer operational

10. Existing FELs Proposed FELs W.B. Colson - THPOS58

11. Storage-Ring FELs The ELETTRA Storage Ring FEL

12. Storage-Ring FELs Critical Issues: • Longitudinal stability of electron beam:rountrip time ofthe optical cavitymust match thebunch-spacing M. Trovò THPOS09

13. Storage-Ring FELs Critical Issues: • Longitudinal stability:rountrip time of the optical cavity must match the bunch-spacing(stability of RF frequency and longitudinal modes) • Transverse stability: wavelength Gain typically micron accuricy required

14. S-ACO: TEM00 TEMnn more stability more gain heat-load Storage-Ring FELs Critical Issues: • Longitudinal stability:rountrip time of the optical cavity must match the bunch-spacing • Transverse stability: wavelength Gain Transverse Cavity Stability!

15. Cavity Stability • Storage-ring FELImportant issue:e.g., 1 W of average output power  200 W of intra-cavity power • High average-power IR FELe.g., JLAB FEL: > 10 kW average output power, i.e., more than 100 kW average intra-cavity power. ref: TUCOS02, TUBOS03

16. acceleration deceleration High average-power lasers: FEL challenge (JLAB) • FELs need a high peak current. • a factor of 4 growth in the longitudinal emittance due to space charge. • longer electron bunches in the injector can reduce space charge effects but reduces the machine acceptance. • Halo loss initially limited the average current. • Resonator FELs need a high average current • Bunch spacing must match the cavity length. Beam break-up limits avarege current 3 mA  8 mA

17. High average-power lasers: FEL challenge (JLAB) • Beam break-up • Cavity design: suppression of HOMs • Clever optics (damping of transverse motion) • Nice presentation by T.I. Smith: WEBOS03 Beam break-up limits avarege current

18. High average-power lasers: FEL challenge (JLAB) • Challenges of recovering the beam • RF kicks reversed for recovered beam • No two pass BPMs • Chromatic effects lead to betatron mismatch, causing beam loss. • Need energy/phase correction to third order (octupoles are required) • Operating close to crest does not provide enough footroom.

19. Single Pass Devices Start-Up from Noise (SASE)

20. Single Pass Devices Controlled Startup (SEED)

21. Single-Pass Devices • Startup from noise (auto-start): SASE • Relatively easy • Flexible for wavelength tuning • High output power • Spiky • Seeded startup • Improved spectral purity • Suppression of spiking • Control over the μ-pulse duration • (Longitudinal jitter defined by seed)

22. Seeded FEL: HGHG (BNL) HGHG 0.23 nm FWHM Intensity (a.u.) SASE x105 wavelength (nm) Courtesy Li Hua Yu (BNL)

23. Single-Pass Devices • Startup from noise (auto-start): SASE • Relatively easy • Flexible for wavelength tuning • High output power • Spiky • Seeded startup • Improved spectral purity • Suppression of spiking • Control over the μ-pulse duration • (Longitudinal jitter defined by seed) LCLS, TTF, X-FEL, SPARC, ….. BESSY, FERMI, TTF, LUX, MIT, ……. LCLS, X-FEL, ……

24. Single-Pass Devices Courtesy T. Shintake, RIKEN/SPring8, MOBIS01

25. Single-Pass Devices Storage Ring vs. LINAC Courtesy T. Shintake, RIKEN/SPring8, MOBIS01 - MODIFIED

26. Single-Pass Devices Beam Energy Stability Courtesy T. Shintake, RIKEN/SPring8, MOBIS01

27. Single-Pass Devices Sources of jitter • AC line fluctuationspower supply fluctuations: gun, RF, orbit, ….. • Switch-tune pulse-to-pulse jitter • Switching noise fluctuations • AD/DA digitizing noise • Temperature fluctuationsLINAC, electrical circuits …. • Ground motionnatural, human activity • Feed-back is mostly not possible • Feed-forward: • between micro-pulses • between macro-pulses

28. Single-Pass Devices Electron-Beam Stability • Transverse stability • Low energy (injector): transverse emittance dilution(depends on the RF-frequency: X-band more sensitive than L-band) • Beam break-up at high average current operation • High energy (undulator): gain reduction and wavelength jitter • Longitudinal stability (bunch / RF phase) • Energy fluctuations • Current fluctuations  FEL gain fluctuations • Temporal jitter with synchronized sources typically a (few) micron orbit accuracy Talking about femto-second and atto-second sources locked to seed laser

29. Single-Pass Devices Transverse Orbit / Undulator • General guideline (not necessarily correct): • Control orbit up to 10 % of the beam diameterεn = 10-6 m rad, E = 2.5 GeV, β = 10 m  Δx,y = 4.5 μm (BESSY)εn = 10-6 m rad, E = 20 GeV, β = 10 m  Δx,y = 1.6 μm (XFEL)εn = 10-7 m rad, E = 6.0 GeV, β = 7 m  Δx,y = 0.8 μm (LEG) • Sextupole fields of undulator may cause tougher constraints, specifically for short-period, small-gap undulators. • N.B. CSR in magnetic bunch-compressors may kick parts of the electron bunch off-axis in the horizontal plate.

30. Courtesy T. Shintake, RIKEN/SPring8, MOBIS01

31. SCSS undulator concept Courtesy T. Shintake, RIKEN/SPring8, MOBIS01

32. Lsat ≈ 15 – 20 Lg z1 z2 Lg Lg = z2 – z1, P(z2)/P(z1) = e Gain Length λ

33. Single-Pass Devices Transverse Orbit / Undulator • Electron beam orbit must be controlled over a few gain-lengths. • Within this distance: gain reduction • Over larger distances the overlap between optical field and the electron beam may disappear. In such a case, the micro-bunching will re-initiate the FEL process over a few undulator periods only. Y.-C. Chae et al (APS) MOBOS03  T. Tanaka, et al., FEL 2003

34. isochronous bend ħω bunching radiation High-Gain Ring FEL • Problems, e.g.,: • Alignment tolerances for single undulators • Isochronous bend: λ < 50 nm • Stable saturation • Beam break-up in linac G1 roundtrip > 1 High-Gain Ring FEL N.A. Vinokurov, A.N. Matveenko – THPOS45